Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typical submersible pumping systems include a number of components, including one or more fluid filled electric motors coupled to one or more high performance pumps located above the motor. When energized, the motor provides torque to the pump, which pushes wellbore fluids to the surface through production tubing. Each of the components in a submersible pumping system must be engineered to withstand the inhospitable downhole environment.
Most wells include a casing that extends along the inside of the wellbore to maintain the structural integrity of the wellbore and to isolate the introduction of fluids into the well. “Perforations” are formed through the casing at desired locations to permit the ingress of fluids from a producing formation into the casing. In many cases, the submersible pumping system is positioned above the perforations in the wellbore. By positioning the submersible pumping system above the perforations, a cooling effect is achieved as fluid drawn into the pump passes by the motor. In installations where insufficient fluid is available to provide this cooling effect, the electric motor may overheat and fail.
There are however, recognized benefits to installing at least a portion of the submersible pumping system below the perforations in what is occasionally referred to as a “sumped” position. By placing at least the intake of the pump below the perforations, the operator is able to maximize wellbore drawdown, which can increase the production of fluids from the well. In certain wells, the placement of the intake below the perforations also decreases the gas content present in the influent to the pump. As two-phase fluids enter the well through the perforations, lighter gaseous components tend to rise as the heavier liquid components fall. Placing the intake of the pump below the perforations enhances gravity separation and decreases the gas content in the pump influent. Reducing the gas content in the influent decreases the risks of gas locking and generally improves the efficiency of the submersible pumping system.
The primary problem associated with placing the submersible pumping system below the perforations is the lack of cooling provided by the movement of fluid over the electric motor. When the submersible pumping system is placed below the perforations, fluid entering the well through the perforations may be drawn into the pump intake without passing over the motor. In this way, the fluid around the motor may become relatively stagnant and unable to provide sufficient heat dissipation.
Manufacturers have used several methods to overcome this problem. The most common method for increasing flow around the electric motor is through the use of a shrouded intake. An intake shroud typically includes a closed end above the pump intake and an open end adjacent the bottom of the motor. As fluids are drawn into the wellbore through perforations, the fluids are conducted around the exterior of the motor by the shroud. While generally effective at providing a fluid flow around the motor, the shroud requires additional space between the submersible pumping system and the well casing and may present an undesirable pressure drop under certain conditions. Furthermore, the cooling effect provided by the shroud is dependent upon the availability of adequate liquid production into the wellbore. In marginal wells or wells with a high gas-fraction, the lack of sufficient quantities of liquid will reduce the cooling effect provided by a shrouded solution. There is, therefore, a need for an improved motor cooling system that overcomes the deficiencies of the prior art.